A unique preexcitation pattern related to an atypical anteroseptal accessory pathway

Accessory atrioventricular pathways have traditionally been classified by anatomical location to four areas, namely anteroseptal, posteroseptal, and right and left free walls. Each of these have been associated with a relatively distinct preexicitation pattern electrocardiographically. We describe a patient with a unique ECG pattern suggesting preexicitation to the right ventricular outflow region. Preoperative and intraoperative electrophysiological testing confirmed the presence of an accessory pathway with an atrial insertion site near the His bundle, decremental anterograde conduction, and a ventricular insertion site in the upper part of the interventricular septum. Operative ablation near the atrial insertion site eliminated preexicitation.